The treble took the Argentine to within three goals of becoming the Club's all-time leading goal-scorer with Aguero now having bagged a remarkable 175 goals since joining City in 2011.
Gundogan, who made a welcome return to action as a second half substitute after nine months on the sidelines, described Aguero as a 'goal-scoring machine'.
And Sterling, who rounded off a magnificent City performance by slotting home from the penalty spot, echoed the German's praise of Aguero's incredible goal-scoring prowess.
"He's very special, just a goal machine," said Gundogan. "His last goal was incredible, it showed all his talent and how powerful he is. We need him and hopefully he'll score a lot more goals."
Sterling was also hugely complimentary, adding: "You can see every game that if you give Sergio a chance, nine times out of ten it's a goal."
The demolition job on the previously unbeaten Watford rounded off a perfect week for Pep Guardiola's side which saw them score 15 goals and concede none in the course of wins over Liverpool, Feyenoord and the Hornets.
And both Gundogan and Sterling believe there is more to come.
"I don't know if it was perfect - after a tough week with the Champions League and the travelling, the way we played was just amazing. We just need to continue like that," added Gundogan.
"Everyone's buzzing. It was a great performance," said Sterling. "We got the ball and we were really clinical today, that's what the manager wants and that's what we were."